High brightness light emitting diodes (HBLEDs) offer enhanced energy efficiency thus making them suitable for specialty lighting applications. An LED device is usually composed of the LED chip fabricated onto a substrate and then encapsulated by a material acting as a lens and a luminescence conversion member. The following are the operational requirements of a material to be utilized as an encapsulant of LEDs: optical clarity, high temperature resistant, UV resistant, high refractive index and good adhesiveness toward LED packaging materials.
Encapsulant materials must be optically transparent (greater than 90% transmittance) and should be able to withstand high temperatures, for extended periods of time, without degradation in mechanical and optical performance. The LED device encounters high temperature conditions during the device fabrication (soldering up to 260° C.) and during the actual device operation (around 150° C. for thousands of hours).
Epoxy resins have conventionally been used as a transparent resin for the encapsulation (1, 2). Also, PMMA (polymethylmethacrylate-PMMA), polycarbonate, and optical nylon have been used. However, optical properties of such conventional resins degrade over time. Coloration, or “yellowing”, occurs either by heat induced degradation (heat resistance) or via prolonged irradiation with short wavelength light (ultraviolet-resistance). The degradation results loss of mechanical and chemical properties. Due to the loss of physical properties water enters from the encapsulated portion to negatively effect the performance of the LED, and the resin discolors by ultraviolet light emitted from LED to decrease light transmittance of the encapsulated portion. Mechanical degradation of the encapsulant also results in cracking, shrinking and/or delamination from the substrate. Thus, it is desirable to have an encapsulant system that is tough and flexible enough to serve as a mechanical support for the LED component, and relieve internal stress during the device fabrication (prevent damage to LED chip or wires), and during temperature cycling (expansion and contraction of materials with different thermal expansion coefficients).
To overcome the above problems, a fluorine-containing cured product in transparent encapsulation of an emission element has been proposed (3). Although this fluorine-containing cured product has excellent colorless transparency, light resistance and heat resistance as compared with the epoxy resin, it has poor adhesion to LED's and it is liable to peel from the material to be encapsulated. The LED is generally made of the following materials (GaN chip, phosphors) and has higher refractive index in the visible light region from 2.5 to 3.0. The fluorine-containing cured encapsulant has low refractive index of light when compared to GaN, phosphor components in the same optical wavelength. Therefore, the pick-up efficiency of light in the same wavelength region has not always been sufficient in the fluorine-containing cured product.
To solve the above problems, LED encapsulated with a glass prepared with a sol-gel method was developed (4). This encapsulant material makes it possible to reduce hygroscopicity and decrease discoloration, and improve heat resistance. However, with sol-gel glass, fine pores are liable to remain and cracks can be easily generated. When water enters the fine pores or crack sites, it affects the performance of the LED. In general, the glass has poor adhesion when compared with a resin.
It has also been proposed that a low melting glass is heat melted, and LED is transparently encapsulated with the melt (5). However, where a low melting glass is generally heat melted, it is necessary to heat the glass to a temperature from 400 to 700° C. Therefore, a phosphor used in LED may undergo heat deterioration.
To overcome those problems, a silicone resin (polyorganosiloxane) having excellent heat resistance and ultraviolet resistance was used as a substitute of the epoxy resin. However, silicone resins up to now tend to scar easily, and are not yet sufficient when considering the combined characteristics: adhesion, colorless transparency, heat resistance, resistance to moist heat and UV tolerant (5, 6, 7, 8, 9).
With the recent development of GaN-based devices which emit short wavelength radiation such as blue light or ultraviolet light, and subsequently white light by combining these light emitting diodes with fluorescent phosphors, the material requirements for the encapsulant have significantly increased. Materials should be able to withstand exposure to radiation of high intensity UV light and temperature up to around 200° C. without degradation in optical and mechanical properties.
Therefore, there is a need for robust LED encapsulants with superior optical clarity, high temperature-resistance, UV-resistance, higher refractive index, and with variable elastic properties including adhesiveness toward the materials used on the surface of the LED packages (10, 11). The present invention allows such properties to be achieved. There is also a need for LED encapsulants with varying mechanical properties, without sacrificing their optical clarity, high temperature-resistance and UV-resistance. The present invention allows such properties to be achieved, as well.